The invention relates to a self-supporting free standing staircase including a center post or spar which is comprised of individual structural elements having treads fixed thereon. Each of the individual structural elements has at one end a top annular surface area and at the other end an annular bottom surface area. These two surface areas are tightly secured by means of screws.
In staircases of this type, the individual structural elements are required to be tightly secured together to insure the safety of the stairway with regard to its load capacity, as well as the prevention of any possible rotational movements of the structural elements. At the same time, it is desirable for the structural elements to be adjustable as to their angular positions to one another so that the stairway may be given, as far as possible, the desired curvature.
In the staircase disclosed by the German Pat. No. DE-PS 1,685,613, the structural elements consist of horizontal hollow profiles which are secured together at their ends by vertical clamping screws. To obtain the desired step rise, a spacer sleeve is positioned between the tightened ends of the hollow sections. To facilitate the attachment of the tread boards, the rectangular profile sections are provided with laterally extending brackets. This construction is material and labor intensive because a great many different parts have to be assembled and adjusted. A serious disadvantage is that the clamping screw, which is positioned substantially centrally with respect to the vertical spacer sleeve, requires considerable prestressing to insure a uniform widespread load distribution over the joint surface areas of the pressure tubing, and simultaneously, to prevent a lifting of the pressure tubing.
Similarly, the aforementioned disadvantages are also inherent in the prior art construction disclosed in the German Published Patent Application DE-OS No. 2,623,523, which basically covers the construction discussed previously and is distinguished from it essentially only by the different construction of the spacer sleeve. This spacer is made adjustable in its length by a thread so that the step rise can be adjusted.
German Pat. No. DE-PS 2,060,480 discloses a self-supporting staircase in which the individual members of the center pole or shaft are made of reinforced (steel) concrete and are directly secured to each other by a single clamping screw to facilitate their rotatability. Also in this prior art construction, the stresses in the region of the surface areas which are tightly fastened together, and also in the clamping screw, are considerable. The load capacity of the stairway is on the whole reduced.
To overcome these problems, German Pat. No. DE-PS 2,129,753 proposes a supporting structure for a flight of stairs in which the individual elements of the center post or shaft, which are made of steel, have tubular sections welded thereto which are adapted to be telescopically inserted into each other. At the desired depth of insertion and mutual rotational position, these metal tubing sections are clamped together by balls encased in holes in the inner tube and pushed by a tapered pin against the outer tube section.
This construction is extremely involved and thus expensive. Moreover, it has the disadvantage that the necessary clearance or play between the two telescoping tubular sections tends to be cumulative with the result that a sagging of the staircase may occur. It has the further disadvantage that the specific area pressure in the region of the balls is significant. In view of the necessary clearance between the tube sections it will be evident that the firm connection between the individual elements is established primarily by the balls, and that there is danger that the strong specific surface pressures may cause a flowing of the material and consequently, a relaxation or loosening of the connection. Furthermore, since the device consists of a multitude of parts, it is not impossible that some of them may be lost before the installation of the staircase is completed, especially the balls, unless special precautions are taken to retain the balls in the holes in which they are encased.
German patent No. DE-PS 1,683,276 discloses a self-supporting staircase having individual elements in the shape of supporting plates to the ends of which are welded bolt type projections of a relatively small diameter. The ends of these bolts are bevelled and are adapted to mate with similarly bevelled bores in which they are retained by relatively small screws. This bevelled fit makes this structure relatively complicated. Moreover, considerable stresses arise in the regions of the bevelled seats and also in the lock nuts, so that this structure is not suitable for conventional staircases. In addition, an angular orientation of the structural elements poses problems because the bevelled fit becomes so firmly interlocked at the slightest tightening of the lock nut that an angular adjustment is no longer possible. A further disadvantage is that an adjustment in the step rise is likewise impossible.